Default HTML Language Level

Normally, an HTML or an XHTML file has the <!DOCTYPE> declaration which states the language level
of the used in the source code from the file. This language level is used as a standard against which the contents of the file are validated.
If an HTML or an XHTML file does not have a <!DOCTYPE> declaration, the contents of the file will be validated
against the default standard (schema).
In the Default HTML Language Level area, choose the default schema
to validate HTML and XHTML files without a <!DOCTYPE> declaration.
The available options are:

HTML 4 or HTML 5:
Choose one of these options to have files treated as HTML 4 or HTML 5 and validated against one of these standards.

Other doctype:
Choose this option to have HTML files by default validated agains a custom DTD or schema
and specify the URL of the DTD or schema to be used.

Note that code completion is available in this field:
press Ctrl+Space to see the list of suggested URLs.